The present invention relates to a process for purifying silicon, and more particularly to a process utilizing the principle of segregation solidification for purifying metallic silicon for industrial uses, such as crude silicon having a purity of about 98 to about 99%, to obtain silicon having a high purity of at least 99.9%.
The percentages as used herein and in the appended claims are all by weight.
It is necessary to use silicon having a high purity of at least 99.9% as polycrystalline silicon for solar cells and also as polycrystalline silicon for preparing single-crystal silicon for semiconductors. Such high-purity silicon is prepared by purifying crude silicon for industrial uses having a purity of 98 to 99% and commercially available. Conventionally, chemical or metallurgical processes are thought useful for purifying such crude silicon. The chemical process generally comprises the steps of reacting crude silicon with hydrochloric acid to obtain trichlorosilane, purifying the trichlorosilane, reducing the purified silane with hydrogen and causing high-purity silicon to separate out on a red-hot rod of high-purity silicon. However, the conventional chemical purification process which includes many steps has the problem of being cumbersome to practice and costly. For metallurgical purification, the principle of segregation solidification is thought useful for crystallizing high-purity silicon from a melt of crude silicon in molten Sn-Pb alloy, Al alloy or Al. The conventional metallurgical purification process nevertheless requires a very long period of time for causing solidification in equilibrium, is low in productivity and has not been practiced on a commercial scale.